The present invention relates to a method of producing internal screw threads, especially in metals or their alloys and particularly in such high-strength alloys as are commonly used, inter alia, in dental prostheses and especially utilizing casting techniques for the production of such dental prostheses.
In the formation of dental prostheses, for example crowns, plates and multiple-tooth sets it is common practice to utilize high strength metal alloys and to provide in such metal bodies, internal screw threads which serve for connection purposes and for fastening to the body. The formation of such internal screw threads in high strength metal bodies has, in the past, posed a problem.
On the one hand, because of the metal used, usually a high strength alloy like, for example, a chromium-cobalt alloy, the Vickers hardness can range up to 500. In such materials the cutting of screw threads, for example, to a depth of 2 mm as is generally required, is particularly difficult. On the one hand, threaded bores are often required in interior regions in dental prosthetics and it is frequently impossible to provide sufficient room for a thread-cutting tap in the regions where such internally threaded bores are required.
To overcome this problem, it is known to provide internally threaded sleeves which are mounted in the casting mold and are secured in the solid body by embedding the sleeves in the hardened melt. A drawback of this technique, however, is that the sleeves themselves require significant space which is not always available. In addition, the melting point of the sleeve must be higher than that of the metal in which the sleeve is embedded and that means that different metals may have to be used in the dental prosthesis which is, as a rule, undesirable.
It is the principal object of the present invention to provide an improved method of making internal threads in a cast metal body which is greatly simplified by comparison to earlier techniques and elements the problems hitherto encountered with conventional thread cutting methods.
Another object of the invention is to provide a method of making a dental prosthesis which obviates drawbacks of earlier techniques.
These objects are achieved, in accordance with the invention, by a method of forming an internal screw thread in a metal body which comprises the steps of:
(a) positioning an externally threaded pin in a casting mold at a location at which an internal screwthread is desired;
(b) filling the mold with a melt of the metal of the body whereby the melt surrounds the pin;
(c) hardening the metal in the mold to form the metal body and embed the pin therein; and
(d) thereafter removing the pin from the hardened metal of the body, thereby leaving an internal screwthread therein.
As applied to the making of a dental prosthesis the method can comprise the steps of:
(a) forming an externally threaded pin with a cylindrical shaft, an externally threaded portion at an end of the shaft, and a cylindrical projection of a diameter less than that of the externally threaded portion extending axially from the externally threaded portion;
(b) positioning the externally threaded pin in a casting mold having a configuration of a dental prosthesis at a location at which an internal screwthread is desired;
(c) filling the mold with a melt of a high-strength metal dental-prosthesis alloy whereby the melt surrounds the pin;
(d) hardening the metal in the mold to form a metal body of the dental prosthesis and embed the pin therein; and
(e) thereafter removing the pin from the hardened metal of the body, thereby leaving an internal screwthread therein.
The projection of the pin forms a cylindrical pocket aligned with the internally threaded bore for receiving a synthetic resin.
More particularly, the objects of the invention are achieved by providing at each location in the mold at which an internal thread is required, a respective pin formed with an outer thread and which projects into the mold cavity so that it is embedded in the hardened metallic melt, the threaded pin being subsequently removed from the hardened body.
With the invention, any machining of the high strength metal is obviated and none of the force required for thread cutting is applied to the dental prosthesis product. The threaded pin can be positioned with high precision before filling of the mold and without any expenditure of force.
According to a feature of the invention, the threaded pin can be composed of a ceramic or some similar material resistant to high temperatures, i.e. a refractory material.
In order to expose the internal screw thread so that it can be used for connection purposes, the threaded pin must be removed and this can be accomplished in a simple manner, just by unscrewing it from the body.
If this is not possible for some reason or another approach may be desired, the threaded pin can be removed after hardening of the metal melt by sandblasting or drilling it out.
Finally, it has been found to be advantageous to provide the threaded pin at one end with a cylindrical shaft from which the externally threaded portion extends while, at the opposite end of the externally threaded portion, a small diameter cylindrical projection is formed. The cylindrical projection can have a diameter less than that of the threaded portion. The cavity formed by the projection can serve to receive the synthetic resin material in a cartridge or body of the synthetic resin which lies ahead of the free end of any screw which is later threaded into the internally threaded bore.
The synthetic resin material forms a bonding agent which prevents loosening of the screw by increasing the friction with which it is held. The cylindrical shaft serves to hold the threaded pin in the refractory mass forming the mold.